The disclosed embodiments relate to magnetic resonance imaging systems, and to a local coil for a magnetic resonance imaging system.
Magnetic resonance imaging (MRI) is a technology in which the phenomenon of magnetic resonance is utilized for the purpose of imaging. The main principles of magnetic resonance are as follows. Where an atomic nucleus contains a single proton (e.g., the nuclei of the hydrogen atoms present throughout the human body), the proton exhibits spin motion and resembles a small magnet. Moreover, the spin axes of these small magnets lack a definite pattern. If an external magnetic field is applied, the small magnets are rearranged according to the magnetic force lines of the external field. The magnets line up in one of two directions, either parallel or anti-parallel to the magnetic force lines of the external magnetic field. The direction parallel to the magnetic force lines of the external magnetic field is called the positive longitudinal axis, while the direction anti-parallel to the magnetic force lines of the external magnetic field is called the negative longitudinal axis. The atomic nuclei only have a longitudinal magnetization component, which has both a direction and a magnitude. A radio frequency (RF) pulse of a specific frequency is used to excite the atomic nuclei in the external magnetic field such that the spin axes of the nuclei deviate from the positive longitudinal axis or negative longitudinal axis, giving rise to resonance. This is the phenomenon of magnetic resonance. Once the spin axes of the excited atomic nuclei deviate from the positive or negative longitudinal axis, the atomic nuclei have a transverse magnetization component. Once emission of the RF pulse has ended, the excited atomic nucleus emits an echo signal, gradually releasing the absorbed energy in the form of electromagnetic waves. The phase and energy level of the nucleus both return to the pre-excitation state. An image is reconstructed by subjecting the echo signal emitted by atomic nuclei to further processing, such as spatial encoding.
An MRI system includes more than one type of coil, such as a body coil covering the entire body and a local coil covering just part of the body. Local coils with a receiving antenna are widely used in MRI systems. Local coils are suitable for various parts of the body of different sizes, and provide a good signal-to-noise ratio. Local coils may also have multiple uses. For example, flexible coils manufactured by Siemens AG may be used for magnetic resonance imaging of various parts of the body, including the chest, abdomen, elbow, knee, ankle and head.
A tuning/detuning circuit in a local coil is incorporated into the coil to safeguard patient safety and the reliability of the coil itself. The tuning/detuning circuit uses a capacitor and an inductor in the antenna part of the local coil to form a parallel resonant circuit. The tuning/detuning circuit also includes a PIN diode. The tuning/detuning circuit forms a loop and resonates only when the PIN diode is conducting. When the local coil is not operating, the diode in the tuning/detuning circuit is switched on with a current of 100 mA to create resonance. The resonance establishes an open circuit in the antenna part that produces a detuning effect. However, current technology has the following shortcomings:
1) Each local coil requires a 100 mA current. An MRI system is often equipped with eight interfaces, each of which is equipped with eight detuning control lines. The total current involved is thus 8*8*100=6400 mA.
2) An additional pin multiplexing circuit is used when two or more local coils are used to multiplex a pin due to a shortage of detuning control lines. The pin multiplexing circuit controls a series connection of the diode when 100 mA is used for detuning, and controls a parallel connection of the diode when −31 V is used for tuning. When a diode multiplexing circuit is used, one of the diodes is unable to use an RF ground pin to form a DC loop because a diode is used to block a DC signal when 100 mA is used for detuning. The 100 mA DC flows in through a first local coil, but flows back via the ground line of the last local coil. Consequently, in spatial terms the 100 mA current travels a very large loop, which may affect the B0 or B1 field if not addressed appropriately. Furthermore, the number of diodes and inductors that may be connected in series in a diode multiplexing circuit is limited because the voltage of a constant current source is limited.